Ammonium nitrate combustion catalyst



Patented Dec. 4, lQfiZ tree 3,067,975 AMMONIUM NTTRATE COMBUSTHQN CATALYST Charles J. Korpics, Chicago, Ill., assignor to Standard Oil Company, Chicago, 15]., a corporation of Indiana No Drawing. Filed Nov. 36, 196i), Ser. No. 72, 9 4- Claims. (Cl. 149-19) This invention relates to an ammonium nitrate-type propellant composition containing a catalyst for promoting the combustion of such composition.

In gas generation and rocketry usages it is necessary that the gas affording composition develop gas at a uniform rate; i.e., burn at a uniform rate. In ammonium nitrate compositions which consists essentially of ammonium nitrate particle-s and an oxidizable organic material which permits the shaping of the composition into a definite configuration or grain, it is necessary to promote the combustion of the composition by the use of a combustion catalyst. Commonly used catalysts are the inorganic chromium compounds, particularly ammonium dichromate and the Prussian blues. These and other heavy metal catalysts have the drawback of forming solid oxide products in the combustion gases which cause nozzle erosion which erosion results in erratic change in gas pressure within the rocket motor.

It has been found that a suitable ammonium nitratetype composition for gas generator and rocketry usage is obtained with ammonium nitrate as the predominant component, between about 10 and 40 weight percent of oxidizable organic material and, as the combustion catalyst, between about 0.1 and 10 weight percent of an alkali metal salt, of ethylenediaminetetraacetic acid (ethylenedinitrilotetraacetic acid). The salts may be mono-, di-, trior tetra-alkali metal. These salts are nontoxic, noncorrosive, easy to handle, and are effective as burning catalysts at low concentrations in an ammonium nitrate-type composition. In addition, they exhibit good storage stability and cause relatively little nozzle erosion.

Suficient catalyst must be introduced into the composition to promote the burning of the composition. The amount of catalyst used is also influenced by the rate of burning desired. The more catalyst present, the faster the combustion of the composition. (It is to be understood that the burning rate is also affected by the particular oxidizable organic material present.) In general, the composition will contain between about 0.1 and 10 weight percent of the catalyst. (Hereinafter all percentages are to be understood as weight percent.) With the thermoplastic matrix formers or binders obtained from cellulose esters and oxygenated hydrocarbon plasticizers there-for, between about 1 and 7% of catalyst produces satisfactory burning rates for typical military gas generation and rocketry usages; more usually l4% is present.

The improved composition of the invention contains ammonium nitrate as the major component. The ammonium nitrate may be ordinary commercial ammonium nitrate such as is used for fertilizers. This commercial grade material contains a small amount of impurities and the particles are usually coated with moisture-resisting material such as parafiin wax. Military grade ammonium nitrate which is almost chemically pure is particularly suitable. The ammonium nitrate is preferably in a finely divided particulate form which may be either produced by prilling or by grinding. The ammonium nitrate is the major component of the gas-generator composition and usually the composition will contain between about 55% and 80% of ammonium nitrate.

In order to permit the shaping of the ammonium nitrate composition into definite configurations, a matrix former or binder material is present. When ammonium nitrate decomposes, free-oxygen is formed. Advantage of the existence of this free-oxygen is taken, and oxidizable organic materials are used as the binders. These oxidizable organic materials may contain only carbon and hydrogen, for example, high molecular weight hydrocarbons such as asphalts or residuums, and rubbers, either natural or synethetic. Or, the oxidizable organic material may contain other elements in addition to carbon and hydrogen, for example, as in Thiokol rubber and neoprene. The stoichiometry of the composition is improved, with respect to smoke production, by the use of oxygenated organic materials as the binders. The binder or matrix former may be a single compound such as a rubber or asphalt or it may be a mixture of compounds. The mixtures are particularly suitable when special characteristics are to be imparted to the grain which cannot be obtained by the use of a single compound.

The multi-component binder or matrix former commonly consists of a polymeric base material and a plasticizer therefor. Particularly suitable polymeric base materials are cellulose esters of alkanoic acids containing from 2 to 4 carbon atoms such as cellulose acetate, cellulose acetate butyrate and cellulose propionate; the polyvinyl resins such as polyvinylchloride and polyvinyl acetate are also good bases; styrene-acrylonitrile is an example of a copolymer which forms a good base material; polyacrylonitrile is another suitable base material. In general, the binder contains between about 15% and 45% of the particular polymeric base material.

The plasticizer component of the binder is broadly defined as an oxygenated hydrocarbon, i.e., where the oxygen is in chemical combination. The hydrocarbon base may be aliphatic or aromatic or may contain both forms. The oxygen may be present in the plasticizcr in ether linkage and/or hydroxyl group and/or carboxyl groups;

also the oxygen may be present in inorganic substituents, particularly nitro groups. In general, any plasticizer which is adopted to plasticize the particular polymer may be used in the invention. Exemplary classes of plasticizers which are suitable are set out below. (It is to be understood that these classes are illustrative only and do not limit the types of oxygenated hydrocarbons which may be used to plasticize the polymer.)

Di-lower alkyl-phthalates, e.g., dimethyl phthalate, dibutyl phthalate, dioctyl phthalate and dimethyl nitrophthalate.

Nitrobenzenes, e.g., nitrobenzene, dinitrobenzene, nitrotoluene, dinitrotoluene, nitroxylene, and nitrodiphenyl.

Nitrodiphenyl ethers, e.g., nitrodiphenyl ether and 2,4-

dinitrodiphenyl ether.

Tri-lower alkyl-cltrates, e. g., triethyl citrate, tributyl citrate and triamyl citrate.

Acyl tri-lower alkyl-citrates where the acyl group contains 24 carbon atoms, e.g., acetyl triethyl citrate and acetyl tributyl citrate.

Glycerol-lower alkanoates, e.g., monoacetin, triace-tin,

glycerol tripropionate and glycerol tributyrate.

Lower alkylene-glycol-lower alkanoates wherein the glycol portion has a molecular weight below about 200, e.g., ethylene glycol diacetate, triethylene glycol dihexoate, triethylene glycol dioctoate, polyethylene glycol dioctoate, dipropylene glycol diacetate, nitromethyl propanediol diacetate, hydroxyethyl acetate and hydroxy propyl acetate (propylene glycol monoacetate).

Dinitrophenyl-lower alkyl-lower alkanoates, e.g., dinitrophenyl ethylacetate, and dinitrophenyl amyloctoate.

Lower alkylene-glycols wherein the molecular weight is below about 200, e.g., diethylene glycol, polyethylene glycol (200), and tetrapropylene glycol.

Lower alkylene-glycol oxolates, e.g., diethylene glycol x0- late and polyethylene glycol (200) oxolate.

Lower alkylene-glycol maleates, e.g., ethylene glycol maleate and bis-(diethylene glycol monoethyl ether) maleate.

Lower alkylene-glycol diglycolates, e.g., ethylene glycol diglycolate and diethylene glycol diglycolate.

Miscellaneous diglycollates, e.g., dibutyl diglycollate, dimethylalkyl diglycollate and methylcarbitol diglycollate.

Lower alkyl-phthalyl-lower alkyl-glycollate, e.g., .ethyl phthalyl ethyl glycollate, ethyl phthalyl ethyl glycollate and butyl phthalyl butyl glycollate.

Di-lower alkyloxy-tetraglycol, e.g., dimethoxy tetra glycol and dibutoxy tetra glycol.

Nitrophenylether of lower alkylene glycols, e.g., dinitrophenyl ether of triethylene glycol and nitrophenyl ether of polypropylene glycol.

Nitrophenoxy alkanols wherein the alkanol portion is derived from a glycol having a molecular weight of not more than about 0. These may be pure compounds or admixed with major component bis(nitrophenoxy) alkane.

A single plasticizer may be'used; more usually two or more plasticizers are used in conjunction. The particular requirements with respect to use will determine not only the polymer but also the particular plasticizer or combination of plasticizers which are used.

In addition to the basic components, i.e., ammonium nitrate, binder and catalyst, the propellant composition may contain other materials. For example, materials may be present to improve low temperature ignitability, for instance, oximes or asphalt may be present. Surfactants may be present in order to improve the coating of the nitrate with the binder and to improve the shape retention characteristics of the composition; Various burningv rate promoters, such as finely divided carbon, which are not considered to be true catalysts, may also be present.

The aromatic hydrocarbon amines improve storage stability and maybe added to reduce gas evolution in high temperature storage. Examples of these aromatic amines are toluene diamine, diphenyl amine, naphthalene diamine, and toluene triamine. In general, the aromatic hydrocarbon amines are used in amounts between about 0.5 and percent. It has been .found that better stabilization is obtained when N-phenylmorpholine is used along with an aromatic hydrocarbon amine. Because of the plasticizing power of the N-phenylmorpholine it is generally desirable to use the aromatic hydrocarbon amines as the primary stabilizing additive and the N-phenylmorpholine in an amount needed to obtain the specific stability. In general, when aromatic hydrocarbon amines are present between about 0.1% and 1% of N-phenylmorpholine will be used.

Broadly the composition will contain between about and 35 weight percent of binder when the polymeric base material is a cellulose ester of an alkanoic acid containing 2 to 4 carbon atoms and an oxygenated hydrocarbon plasticizer therefor. A particularly useful composition consists of cellulose acetate, about 612%; acetyltriethylcitrate, about 6-12%; a mixture having two to four parts of dinitrophenoxyethanol to one part of bis (di-nitrophenoxy)ethane, about 612%; carbon, about 24%; toluene diamine, about 0.5%; N-phenylmorpholine, about 0.5%; and catalyst, about 14%.

TESTS Three compositions were tested for burning rate, storage stability, and other characteristics required by military specifications. Composition A contained no catalyst; composition B cotnained 3.0% and composition C contained 1.0% of tetrasodium salt of ethylenediaminetetraacetic acid.

Each composit on .Was prepared by mixing together for one hour in a laboratory..mixery'a 300-gram batch having ingredients in the proportions indicated in the table set forth below. The mixing temperature was about C. Lacquer grade commercial cellulose acetate analyzing about 55 percent of acetic acid equivalent was the polymer base. Two plasticizers were used. One plasticizer contained about three parts of dinitrophenoxy ethanol and one part of bis(dinitrophenoxy)ethane, obtained by the reaction of dinitrochlorobenzene and ethylene glycol in the persence of aqueous sodium hydroxide solution. The other plasticizer was acetyl triethyl citrate. Toluene diamine and N-phenylmorpholine were used as stabilizers.

After mixing the resulting pasty mass was compression molded into a slab approximately one-half inch in thickmess. The slab was subsequently sawed into strips for the burning rate test and broken into smaller pieces for the storage stability test.

The burning rate'tests were conducted in a Crawford Bomb pressured at 1000 p.s.i.g. and 25 C.

The following table summarizes the ingredients of each composition testedand the results of the burning rate and high temperature stability tests conductedthereon:

Table I Composition A Ingredients, Weight percent:

Ammonium nitrate Cellulose acetate Dinitrophcnoxy ethanol and his (dinitrophenoxwethane Acetyl triethyl citrate Carbon black..- Toluene diainine N phenyl morpholincn Catalyst salt Totals anoncm TEST RESULTS.

BurningRate, inches/sec 0.055 Pressure Exponent O. 60

Thus having described the invention, what is claimed 1. A composiiton consisting essentially of 'between about 0.1'and 10 weight percent, as a'combustion catalyst an alkali metal salt of ethylenediaminetetraacetic acid;. ammonium nitrate as the predominant component; and "between about 10 and 40 weight percent of oxidizable organic bindermaterial.

2. The composition of claim 1 wherein said binder material consists of a polymeric base selected from the class consisting of cellulose esters of alkanoic acids containing from 2 to 4 carbon atoms, polyvinyl chloride, polyvinyl acetate, polyacrylonitrile, and styreneacrylonitrile, and an oxygenated hydrocarbon adapted to plasticize said polymer.-

3. A composition consisting essentially of ammonium nitrate as the predominant component, between about 1 and'7 weight percent of an' alkali metal salt of ethylenediarninetetraacetic acid, between about 20 and 35 weight percent of a binder consisting of a cellulose ester of an alkanoic acid containing 2 to 4 carbon atoms and plasticizer adapted" to plasticize said polymer.

4. A composition consisting essentially of (at) ammonium nitrate, (b) cellulose acetate, about 6-12%, (c) acetyl triethyl citrate, about 6-12%, (d) about'612% of an about 3:1 mixture of dinitrophenoxyethanoland bis(dinitrophenoxy)ethane, (e) carbon, about 24%, (f) toluene diamine, about 0.5%, (g) N-phenylmorpholine, about 0.5%, and (h) the tetrasodium salt of ethylenediaminetetraacetic acid, about l-4%.

No references cited. 

1. A COMPOSITION CONSISTING ESSENTIALLY OF BETWEEN ABOUT 0.1 AND 10 WEIGHT PERCENT, AS A COMBUSTION CATALYST AN ALKALI METAL SALT OF ETHYLENEDIAMINETETRAACETIC ACID; AMMONIUM NITRATE AS THE PREDOMINANT COMPONENT; AND BETWEEN ABOUT 10 AND 40 WEIGHT PERCENT OF OXIDIZABLE ORGANIC BINDER MATERIAL. 